1. Field of the Invention
The present invention relates generally to mobile communications, and in particular to a method for setting up a call requested by a mobile station through indirect access using a base transceiver station of a mobile telecommunication system. More particularly, the present invention relates to methods for setting up a call requested by the mobile station within a specific sector, and where the mobile station then travels to a different sector by the time the base transceiver station has initiated the call setup in a code division multiple access (CDMA) communication technique.
2. Description of the Related Art
A mobile telecommunication system using a code division multiple access (CDMA) technique comprises a plurality of cells each having a base transceiver station (BTS) for providing communication service to mobile stations, a plurality of base station controllers (BSC) for controlling a plurality of BTSs, and a mobile switching center (MSC) for connecting a plurality of BSCs to a public switched telephone network (PSTN).
In such a CDMA system, a forward link channel is a channel which is setup from a BTS toward a mobile station. A reverse link channel is a channel which is setup from a mobile station toward a BTS. Since all forward link channels within a BTS have the same PN offset, a mobile station identifies base transceiver stations with a PN offset.
The mobile telecommunication system enables multiple subscribers to set up calls at the same time by allocating different frequencies for each of the respective subscribers. To make frequency allocation more efficient, a cell division technique is generally implemented where each of the cells of the telecommunication system is divided into sectors. Typically, each cell is divided into three sectors by placing three antennas at the BTS having a 120.degree. angle of separation.
The cell division technique is disclosed in U.S. Pat. No. 5,428,818, entitled "Method and Apparatus For Reducing Interference In A Radio Communication Link Of A Cellular Communication System", wherein the technique entails using a different antenna for each sector (geographic service area) of a cell.
When a cell division technique is implemented, each sector is assigned its own characteristic PN offset and a mobile station within the cell recognizes each sector as a BTS. When the mobile station located in a certain sector of a cell requests a call setup, the BTS tries to set up the call using information from the sector in which the mobile station is located.
With reference to FIG. 1, there is illustrated a procedure for setting up a call after having divided the cell into sectors according to the prior art. As shown, the cell is divided into three sectors, i.e., sector .alpha. 100, sector .beta. 200, and sector .gamma. 300. If a mobile station 2 which is located in one of the sectors, for instance, in sector .gamma. 300, requests a call setup, a base transceiver station 1 confirms whether there is a preamble signal, i.e., a reverse traffic signal which the mobile station 2 transmits to the base transceiver station 1. A traffic channel element of the BTS 1 receives the reverse traffic signal from the mobile station 2 using information about the sector designated by the BTS 1 as having the mobile station 2 therein.
However, according to the prior art, the BTS 1 does not recognize a change of location (for example, from sector .gamma. 300 to sector .alpha. 100) of the mobile station 2, if any, from the time the request for call setup is made to the time that the call setup is accomplished. In particular, if an idle handoff is about to occur, the mobile station 2 may be located in a first sector when it requests a call setup and in a second sector when the mobile station 2 transfers the reverse traffic signal. In such a situation, it is very difficult for the BTS 1 to receive the reverse traffic signal from the mobile station 2 using only information about the first sector, i.e., the sector where the mobile station 2 was located in when it requested the call setup.
In other words, if a mobile station requests a call setup in an original sector (orig_sector) and then moves to a sector other than the orig_sector, a BTS which has only information about the orig_sector cannot receive a reverse traffic signal from the mobile station.
FIG. 2 illustrates movement of a mobile station during a call setup according to the prior art. As shown, a mobile station 2 which is in the access state requests a call setup in a sector .gamma. 300 and a base transceiver station (BTS) 1 receives the request for call setup, processes the call setup using information about the sector .gamma. 300 in which the mobile station 2 is currently located, and assigns a traffic channel.
However, the mobile station 2 which has requested the call setup has already moved to sector .alpha. 100; the mobile station 2 now being designated by reference numeral 3. The prior art BTS 1 searches a reverse traffic signal of the mobile station 3 in sector .gamma. 300 only, and terminates initiation of the call, when no reverse traffic signal is identified.
As stated above, in the case where a mobile station requests a call setup in a region where an idle handoff may occur, a base transceiver station is not able to receive a preamble transmitted from the mobile station, i.e., a reverse traffic signal, using information about the original sector only. Consequently, even though a radio environment is good, the probability of successfully completing the call requested decreases.
In order to solve these problems of the prior art, it is proposed that a mobile station re-request a call setup from the new sector. Then, the BTS would receive a new request for call setup, process the call setup and assign resources accordingly. However, to implement this methodology, the structure and design configuration of the mobile station would need to be changed in order for the mobile station to be able to determine that it has moved to a different sector and a re-request of the call setup is necessary. Accordingly, a need exists for solving the problems of the prior art without changing the structure and design configuration of the mobile station.